Recently, technologies about femto cell for supporting indoor or shadow area which macro cell cannot support are being defined. A femto cell base station is a micromini and low power indoor base station. femto cell is a coverage of a femto cell base station and similar concept with pico cell. Femto cell is evolved more than pico cell. A femto cell base station is a mini cellular base station connected to a broadband router and connects data to backbone network of a mobile communication company trough Digital Subscriber Line (DSL).
A femto cell base station can be established as overlay in macro cell area or as non-overlay in area which a macro base station does not cover.
When a mobile station connected to a macro base station passes area of a femto cell base station established as overlay in macro cell area, it performs handover to the femto cell base station or not.
If a mobile station, which has authority to access to the femto cell base station and whose speed is low, performs handover to the femto cell base station, the mobile station can be served with good signal.
But, a mobile station, which does not have authority to access to the femto cell base station, cannot perform handover to the femto cell base station. If a mobile station whose speed is low performs handover to the femto cell base station, signal overhead generates because the mobile station is served by the femto cell base station in a short time and performs handover after escaping from area of the femto cell base station.
A femto cell base station established as overlay in macro cell area and the macro base station can use same frequency.
If a mobile station connected to a macro base station approach a femto cell base station established as overlay in macro cell area and does not performs handover to the femto cell base station, the mobile station receives interference from the femto cell base station.
In case that a femto cell base station does not use frequency band which is used by a macro base station, inter frequency handover between a femto cell base station and a macro base station occurs. And, a mobile station cannot receive downlink of a femto cell base station and a macro base station at once. And frequency band can not be used efficiently.